As wrapping materials for filling and packing of foods and beverages, chemical products, sundry goods and the like in the prior art, there have been developed substrates with gas barrier properties in a variety of forms that block or shield the passage of oxygen gas, water vapor and the like, in order to prevent alteration or discoloration of the contents.
Typical materials that have been proposed are aluminum foil or metal aluminum vapor-deposited films, that are largely unaffected by temperature or humidity, and although these exhibit highly stable gas barrier properties, when they are incinerated as waste after use, they are poorly suited for incineration and are not easy to dispose of after use, while another problem is that they have low transparency.
As a solution it has been attempted, for example, to use resin films comprising polyvinylidene chloride-based resin, ethylene-vinyl alcohol copolymer or the like, that have barrier properties that block or shield the passage of oxygen gas, water vapor and the like.
However, because polyvinylidene chloride-based resins contain chlorine in the structure, when they are incinerated as waste after use they generate harmful chlorine gas, which is undesirable from the viewpoint of environmental sanitation.
On the other hand, ethylene-vinyl alcohol copolymers have the advantage of both low oxygen permeability and low absorption of flavor components, but when contacted with water vapor their gas barrier properties are significantly reduced. It is therefore necessary at the current time to employ a complex layered structure of an ethylene-vinyl alcohol copolymer as a substrate with a barrier property, in order to block water vapor, and this tends to increase production cost.
This has led to development of substrates having a barrier property comprising a barrier layer that comprises a thin-film of an inorganic oxide such as silicon oxide or aluminum oxide, as a plastic with a gas barrier property, exhibiting a stable high gas barrier property and having transparency.
In addition, in fields that require high-temperature, high-pressure retort treatment or sterilization treatment of foods, drugs and the like, there has been a desire for substrates having a barrier property that is not affected by temperature or humidity and being able stably exhibit higher gas barrier properties, in order to prevent alteration of contents and maintain their functions and properties, and this has spurred development of substrates with barrier properties, having a multilayer structure comprising a barrier layer made of a thin-film of an inorganic oxide such as silicon oxide or aluminum oxide, and a coating film layer with a gas barrier property.
However, plastic substrates that are easily affected by temperature and humidity easily undergo dimensional change, and therefore an inorganic oxide vapor-deposited film layer such as a transparent silicon oxide thin-film layer or aluminum oxide vapor deposition layer that is formed thereover cannot easily follow the expansion and contraction that takes place with dimensional change of the plastic substrate.
Consequently, the phenomenon of interlayer separation often occurs between the plastic substrate and the inorganic oxide vapor-deposited film layer such as a transparent silicon oxide thin-film layer or silicon oxide vapor deposition layer in high temperature, high humidity environments and the like, and cracking or generation of pinholes can also occur.
As a result, the original barrier performance is significantly lost, and it is extremely difficult to retain barrier performance.
When the aforementioned vapor deposition method is used to form a transparent vapor-deposited film of an inorganic oxide, such as aluminum oxide, on a plastic substrate, the method generally employed to obtain high adhesiveness between the plastic substrate and the formed vapor-deposited film layer is inline plasma preprocessing with a parallel flat plate-type apparatus, or modification of the plastic substrate surface by formation of an undercoat treatment layer (see PTL 1 and PTL 2, for example).
However, the commonly used method of inline plasma processing method with a parallel flat plate-type apparatus described in PTL 1 introduces functional groups such as hydroxyl or carbonyl groups into the plastic surface, creating adhesiveness with the vapor-deposited film via the functional groups. However, when adhesiveness is produced by hydrogen bonding with hydroxyl groups, the adhesiveness is notably reduced in the high temperature, high humidity environments required for electronic device use, such as in the case of electronic paper, because of destruction of the hydrogen bonds.
Furthermore, since plasma processing merely passes the film under a plasma atmosphere generated in air, it is currently not possible to achieve sufficient adhesiveness between substrates and vapor-deposited films.
Furthermore, the undercoat treatment method described in PTL 2 is usually carried out by providing an undercoat layer as an adhesive layer on the plastic film surface, and this increases cost due to a greater number of steps during the production process.
A technique for improving adhesiveness is therefore employed, in which the electrode for plasma generation is situated on the substrate side and a reactive ion etching (RIE) system that generates plasma is used for preprocessing (PTL 3).
This plasma RIE method produces adhesiveness by simultaneously generating two effects, a chemical effect that includes introduction of functional groups onto the surface of the substrate, and the physical effect of ion etching of the surface causing fly-off of impurities and smoothing.
In RIE methods, unlike the aforementioned inline plasma processing, adhesiveness is not exhibited by hydrogen bonding and therefore no reduction in adhesiveness is seen in high temperature, high humidity environments.
However, since RIE methods introduce functional groups onto the plastic substrate, the resistance to cold water and hot water that can cause hydrolysis at the interface has still remained insufficient. In addition, in order to obtain sufficient adhesiveness it is necessary for the Ed value (=plasma density×processing time) to be at or above a certain value.
In RIE methods as well, it is necessary for the Ed value (=plasma density×processing time) to be at or above a certain value in order to obtain sufficient adhesiveness. An Ed value at or above a certain value using the same method can be achieved by increasing the plasma density or lengthening the processing time, but increasing the plasma density requires a high output power source, which can increase damage to the substrate, while lengthening the processing time can lower productivity (see PTL 4 and Patent Publication 5).
Furthermore, the following problems may arise, depending on the film-forming method used in combination with the preprocessing.
In vacuum vapor deposition methods, the thin-film formation speed is not slow, but the precision of thin-film homogeneity is poor, leading to poor yields.
In sputtering, despite satisfactory precision of thin-film homogeneity, the thin-film forming rate is very low and productivity is poor.
In thermal CVD processes, a source gas is oxidized and decomposed by the heat energy of the substrate to form a thin-film, and they require the substrate to be at high temperature, and when the substrate is a plastic film, decomposition and oxidation of the plastic film can occur, making it impossible to form a homogeneous thin-film on the plastic substrate.
In methods of forming a vapor-deposited film on a plastic substrate that has been subjected to preprocessing by conventional preprocessing means, problems have arisen such as insufficiency of the barrier property of the inorganic oxide vapor-deposited film formed on the plastic substrate, even if the substrate with a gas barrier property previously had resistance to moist heat, or problems such as insufficiency of adhesiveness between the vapor-deposited film and the plastic substrate.
Also, when a vapor-deposited film is formed by a film forming device combined with the aforementioned preprocessing, formation of the vapor-deposited film is accomplished in a continuous manner but without uniform formation of the vapor-deposited film, and attempts to maintain adhesiveness have resulted in a slower vapor-deposited film-forming speed and lower productivity.
In addition, when using conventional vapor-deposited films there has been limited success in obtaining sufficient resistance to moist heat while maintaining adhesiveness.
A need therefore exists for a transparent vapor-deposited film having high adhesiveness, that can solve the problems encountered with plastic substrates having vapor-deposited films with a barrier property, when an inorganic oxide vapor-deposited film is formed on the surface of a plastic substrate that is being transported as described above, and that allows production of a vapor-deposited film wherein the vapor-deposited film is reliably bonded even at high film-forming speed, while improving the adhesiveness between the plastic substrate surface and the inorganic oxide vapor-deposited film and stably exhibiting barrier performance.
In addition, there is a need for a water-resistant adhesive transparent vapor-deposited film having a vapor-deposited film with reinforced water-resistant adhesion after hot water treatment at 121° C., 60 min, on a plastic substrate having a vapor-deposited film with a barrier property, wherein the adhesiveness between the film and the inorganic oxide vapor deposition layer is not decreased after hot water treatment at 121° C., 60 min.
There is additional need for a highly adhesive transparent vapor-deposited film with resistance to moist heat, on a plastic substrate having a vapor-deposited film with a barrier property, the vapor-deposited film being such that a sufficient gas barrier property is maintained and the adhesiveness between the film and the inorganic oxide vapor deposition layer is not reduced, even after storage for 500 hours in an environment of 60° C.×90% RH (a high temperature, high humidity environment).
There is yet further need for a transparent vapor-deposited film that is suitable for retort purposes.